This invention relates generally to band-stop or band-rejection filters for VHF (very high frequency) and/or UHF (ultrahigh frequency) band, and more particularly, the present invention relates to such a filter having a plurality of series resonance circuits connected by transmission lines.
Conventional band-stop filters used in the VHF-UHF bands usually comprise a plurality of series resonance circuits, where each resonance circuit has a high value of unloaded Q and is of a distributed constant type. The series resonance circuits are connected by transmission lines, and usually the length of each transmission line connected between two series resonance circuits is selected to a value which equals a quarter wavelength of the center frequency in the stop band. The frequency characteristic curve of the insertion loss of such a conventional band-stop filter is substantially symmetrical with respect to the center frequency. When such symmetry of the insertion loss with respect to the center frequency is not required, but when a sharper attenuation characteristic is required, the length of each transmission line between adjacent series resonance circuits may be reduced by 5 to 20 percent from a quarter wavelength as already proposed by the inventors of the present invention. Alternatively, the length of each transmission line may be lengthened from the quarter wavelength to obtain a similar effect. Although such already known band-stop filters can provide an attenuation characteristic which is sharper than that of the conventional band-stop filters having transmission lines whose length equal quarter wavelength, the sharpness is not adequate when a further sharper attenuation characteristic is required.